Television and like apparatus



.1. 1.. BAIRD 1,980,150 TELEVISION AND LIKE APPARATUS I Filed March 2. 1932 3 Sheets-Sheet 1 Nov. 6, 1934.

I I IIIIIIIIIII Nov. 6, 1934. J. 1.. BAIRD TELEVISION AND LIKE APPARATUS 3 Sheets-Sheet 2 Mines;

Nov. 6, 1934. J. L. BAIRD 1,980,150

TELEVISION AND LIKE APPARATUS Filed March 2. 1932 s Sheets-Sheet 5 Mines jrruerzzart Patented Nev. 6, 1934 smvrsron nn LIKE ArPAnATUs John Logic Baird, London, England, assignor to Baird Television Limited, London, England, a

British company Application March'Z, 1932, Serial No. 596,254 In Great Britain March 4, 1931 3 Claims.

This invention relates to systems for the electrical transmission of views, scenes, objects, or optical images of the same to a distance, by means of wires, or by radio transmission; and

5 more particularly to television or like systems by which two or more persons situated at a distance from each other are enabled to see and hear each other simultaneously or selectively.

In twoor multi-way television systems, it is 0 desirable that simultaneous transmission and reception of images of a plurality of objects at the several co-operating stations should take place within the period of the retentivity of vision of each observer.

It has formerly been proposed to accomplish two-way television in which this desirable feature is obtained, in several ways.

For instance, in the specification of British Letters Patent No. 309,965, there is described apparatus for eflecting televisionbetween two stations, comprising at each station a transmitting device and a receiving device, which devices are mechanically connected together and driven by a single motor.

In one embodiment of this apparatus, a disc at each station is provided with two concentric spirals of apertures, lenses, or the like, to serve respectively for transmission and reception.

Further, in such a system, synchronism is judged by the movement or appearance of the image on the receiving screen at either station,

- and is'ochronization and phasing of the mechanism at the two stations may be effected at either station, or at both.

The chief disadvantages of the system disclosed in the specification of British Letters Patent No. 309,965 are that, in order toefiect simultaneous transmission in two directions, either two channels of communication must be 40'employed, or else two effective channels must be provided by superposing one or both of the television signafi upon an appropriate carrier frequency, according to practice known in the art, and that, when the observer at each station looking at the received image of the observer at the other station, he is viewed by the scanning device at his own station rather obliquely, and- -at the other station, which methods consist essentially in the provision of means for reconstituting an image at each station within the solid angle of the scope of the scan at that station, and in irradiating the observer at each sta- 0 tion with invisible rays for the purpose of effecting traversal by the scanning ,device, in order that the observer may be able to view the reconstituted image, without being dazzled.

In previously devised systems, it has also been necessary to use two or more channels of com-' munication between an operator-observer at one station and the corresponding operator-observer at the second station, in the case of: two-way television transmission and reception (see, for instance, the specification of British Letters Patent No. 309,965). This necessity constitutes a serious detraction from the commercial value of such a system when long distances are to be covered by transmission through telephone landlines or submarine cables, for the electrical frequency-attenuation correctors and loading coils,. repeaters and the like, associated with each such transmission circuit, are expensive in initial cost and upkeep. when, on-the other hand, communication is efiected by means of radio-transmitting and receiving apparatus, duplication of the apparatus at the transmitting and receiving stations is almost unavoidable, and two discrete wavelengths must be employed, one for the incoming and one for the outgoing television transmission, with, possibly an additional wavelength, or alternative channel of communication, for speech.

According to the present invention, it is proposed to employ only one channel of communication between the stations of a two-way or multiple-way television system, and to transmit intelligence from each station .discontinuously, the quiescent periods during which one of the stations is not transmitting intelligence being employed for the reception of intelligence from the otherstations of a group of co-operating stations of a multiple-way system and this form of transmission is utilized to provide an improved means, as hereinafter described, for obtaining synchronism between two stations, or

between some or all of a group of co-operating stations.

According to another feature of this invention, we propose to transmit the image signals from each of two co-operating stations directly out of phase, that is to say a high light at one station will, result in an impulsive flow of current in a given direction in the communicating .used in the system.

channel and a' high light at the other station will result in an impulsive flow, not in the opposite direction, but in the same direction; by this method oftransmitting we can obtain very eflicient synchronizing. f

In order that the present invention may be more clearly described and ascertained, reference is made to the accompanying diagrammatic drawings in which:

Figure 1 shows a complete two-way television system.

Figure 2 shows the form of scanning disc Figures 3'and 4 show the relativepositions of the discs at the two stations.

Figure 5 illustrates the method of applying the image signals to the transmission line to obtain eflicient synchronizing.

Figure 6 shows a circuit, the use of which prevents the local image from affecting the local light-relay.

Figure 1 illustrates, by way of example, a twoway television system incorporating a method of maintaining synchronism between co-operating scanning devices at the two stations in accordance with the present invention. In this system each station is provided with a rotary scanning device for the purpose of effecting traversal of an operator thereatand at the same time reconstituting an image for his benefit, according to practice well-known in the art per se. 1

The nature of the various elements at one of the co-operating stations, to which those at'the other exactly correspond, will now be described and their various functions explained.

At the station on the left hand side of Figure 1, element 2 is a scanning disc mounted upon the shaft of a driving motor 28 and provided with a double series, of apertures, each series conforming to a singleconvolution of a spiral to effect traversal according to practice very well known in the art, but the apertures of one series lying upon radii of the disc mid-way-between the radii upon which the apertures of the other series lie. Moreover, the radius of the outermost aperture of one series, hereinafter referred to as the inner series is substantially less than the radius of the innermost aperture of the other series hereinafter referred to as the outer series.

The disc 2 is also shown, in greater detail, in Figure 2.

A light source, preferably comprising an electric arc, 4, is provided, and co-operates with a condensing lens, 6, to illuminate with substantially even intensityan area 'of the disc 2 through which area pass the apertures of the inner series in ordered sequence during the rotation of the disc. Upon the opposite side of the disc 2 from the arc 4 there is a projecting lens 10 which serves to focus an image of each of the apertures of the inner series in turn upon an operator 8 during the passage of the said aperture across the field on the disc illuminated by the are 4 in co-operation with the lens 6.

Co-operating with the outer series of apertures in the disc 2 there is an element 20 comprising a glow discharge lamp of type well-known in the art having a flat rectangular cathode substantially' co-extensive in one sense with the pitch of the spiral to which the apertures of the outer series conform, and in the other-with the width, in the direction of motion of the disc 2, of an image to be reconstituted thereby. Upon the opposite side of the disc 2 from the glow discourse, equal.

charge lamp 20 there is a magnifying lens 22;

which serves to present to the operator 8, an enlargederect virtual image of an area of the disc 2 immediately in front of the glow discharge lamp 20.

The glow discharge lamp consists preferably of a neon-filled tube in which the cathode glow is excited at current densities of and above. about 5 milliamperes over substantially the whole of at least one surface of the rectangular cathode.

A mask or shield 29 serves appropriately to frame those portions of the inner and outer series'of apertures which are to be' operative in scanning and reconstituting respectively.

A photoelectric cell or a group or series of groups of photoelectric cells represented dia- Upon the shaft of the driving motor 28 (or in .modifications upon a shaft geared thereto) is a laminated toothed rotor of ferro-magnetic material 26 of the phonic wheel type, adapted to co-operate with a stator or fixed field winding of any type known in the art. The number of teeth upon the rotor corresponds to and is in a preferred modification of this invention, equal to the number of apertures in one series of apertures in the disc 2: the numbers of apertures in inner and outer series in the disc are, of

It is preferable to pass apolarization current through the stator winding of the phonic wheel and this may conveniently be. the steady plate current of the output valve in the amplifier 24.

In operation, according to the present in vention the system functions in the following" manner. e

The disc at each of the co-operating stations is rotated at a speed of about 750 revolutions per minute, and the optical systems, including the are 4, the condensing lens 6 and the projecting lens 10 and their corresponding equivalents at the other station, are adjusted to yield a scanning beam in which the flux of radiation-is as large as possible. In the case of luminous r'adiation, the flux should be at least one third of a' lumen, and preferably as high'as two or three lumens.

Energy diffused from the surface of the operator 8 becomes incident upon the photo cell or photo cells 12, and is there operative in producing an electric signal representing the optical density of each part of the scanned area in turn- This signal, hereinafter referred to as the first image signal, is passed by way of a suitable thermionic amplifier 14 to a channel of communication 16, which may comprise a pair of wires, a radio link, a submarine cable, or any other suitable transfer means. At the other terminal of this channelis the co-operatingstation of the two way system, comprising elements respectively the same as and similarly positioned to, those at the station above described. From this co-operating station proceeds a signal (hereinafter referred to as the second image signal) of type similar to that derived at the station described above.

Now the mask or shield 29 is so designed that light can become operative in scanning the operator only for those periods'during which an aperture is within a prescribed portion of its ambit, which portion corresponds to half the distance between successive apertures of the inner series. That is, light is allowed to pass to the scanned area while an aperture is passing across the area 36 shown in Figure 2.

It will thus be seen that two signals are super.- imposed upon each other in the channel 18, and it is arranged in accordance with this invention that the efiect of the superimposed signals (that is, the locally-derived and incoming signals) is vectorially additive. tion may best be explained with reference to the curves shown in Figure 5.

Let the curve shown at a represent the first image signal, and that at b the second image signal, each taken over two -complete scanning strips and two intervals.

Let the curve a,'of which abscissae measure time and ordinates amplitude, be taken as a curve of delivery from the output of the ampliher 14, and curve b be taken as a curve of arrival at the same point of signals from the distant station. Then it will be appreciated Iromthe curves that the curve of arrival represents the production of potential diflerences oi thesame sign as that of the potential difference generated by the local signals. Suppose that ex-: actly the same number of signals, each representing a strip of the scan,,is received from the distant station as is generated locally during each second: thenby adjustment of the phase of the local signals it is clearly possible to arrange for the incoming signals just to fill in, as it were, the troughs between the potential impulses due to the locally derived signals.

This means, in accordance with the terminology at present current inthe art, that the strip-frequency fundamental of the incoming television signals is exactly 180 out of phase with that of the strip frequency of the locally derived signals: If, in addition, it is arrangedby suitably balanced amplification, that the amplitudes of the two components thus superposed 180 out of phase are substantially equal, we have the condition illustrated by the dotted curve of Figure 5c, in which the resultant of the two superposed strip-frequency fundamentals is shown to be a straight line-that is, a sensibly constant potential difference,-substantially Cor incident with the datum line that is, of sensibly zero value.

These superposed signals are passed by the thermionic amplifier 18 to the neon tube 20, and also, by a branch circuit in shunt, to a circuit 24, which comprises a thermionic am-v plifier, preferably incorporating a circuit or circuits tuned approximately to the frequency of that component of the television signals which is numerically equal to the number of strips scanned at each station per second. It is, of course, impossible to obtain sharp tuning at such a low frequency because of the inherent resistance of the inductances employed, but this is an advantage, provided that the filtering action of the tuned circuits is adequate to elide components due to details of the configuration in the pictures transmitted. This circuit 24 yields alternating current power to the field circuit associated with the toothed rotor 26, which, it will be remembered, is provided with thesame number of teeth as there are apertures in one of the spiral series of the disc 2, and therefore The nature of this condimaintains a fixed phase relationship with the locally generated strip. frequency component or v with the corresponding component of the incoming signals. This phaserelatlonship may bereckoned as existing between the locally derived signals and the back electromotive force due to therotation of the rotor near the pole pieces of ,the stator, and its valueis such that maximum current occurs in the field coils, due to the local signals, when the pole-tips of the field are midway between'teeth of the rotor. In operation, therefore, since the strip-frequency components of the two sets of signals mutually annul one another, he controlling torque is exerted by the toothed rotor 26 upon the shaft of the motor 28.

If, however, we now relax our hypothetical condition that the incoming signals exactly fit into the troughs of the locally derived ones, as it were, then one set of signals willtend to overtake the other in phase, according to'which of the two scanning devices is running-faster. This state of affairs is illustrated in Figure 5d, wherein the two sinusoidal components of Figure 5c are shown with a phase relationship differing from 180?. A. resultant component is generated of phase midway between those of-the two signal components, and of amplitude dependent upon the degree of departure from the 180 phase difference hereinbefore postulated. This resultant component rises in magnitude with increasing departure from the 180 phase difierence condition (or what amounts to the same thing, with increasing proximity in phase) on the part of the'two strip frequency com- 'ponents.- By means of the selective amplifier 24 this resultant is passed to the field windings of the toothed rotor 26 and there serves to con-' trol its-speed and tends to bring the-speed of the motor 28 to such a value that the 180 phase difference between the incoming and locally derived components is restored and maintained. This speed-controlling effect on the part of the toothed wheel, fed by the resultant above referred to may be simply' explained as follows:- In the normal, or correctly phased, position as previously explained the current maxima (corresponding of course to potential maxima of magnetic intensity in the stator pole pieces, associated with the toothed rotor) due to the locally derived signal, would occur when the stator poletips are mid-way between two rotor-teeth if they were not annulled. If, however, the local rotor tends to overspeed, the incoming signals lag on the local ones and a resultant current is applied to the field coils which tends to diminish the magnetization of the'fieldjust as a tooth of the rotor is approaching a stator pole-tip, and which increases the magnetization above the normal (which may be assumed to be due to135 an applied direct current component in the field windings) soonafter a rotor-tooth has passed a stator pole-tip. This state of affairs corresponds to the application of a retarding torque to thelocal rotor, and tends to restore the motor 28, may have an extra field winding that is supplied with the rectified synchronizing impulses for the purpose of altering the field and hence the speed ofythe motor.

v the same speed and with a phase relationship such that when a spot of light is exploring the top strip of an operatorsimage the disc at the other station is exposing to viewthe top strip of the glowing plate of the light valve 20. :1 v

Figure 2 shows the form of scanning disc used. This has two sets and 32, of spirally arranged apertures so arranged that when an aperture of one set is in either of the areas '34 or 36 (these areas represent the apertures in the. mask 29) an aperture in the other set is not within the other area. 'At the same time it is arranged that an aperture is always within one of the areas. With; this type of disc it is clear that single strips of-thetwo operators images-are transmitted alternately but the present invention is not limited to this extent, for example, 'a plurality of strips of each operator's image may be transmitted alternately tor even a complete image or series of complete images may be alternately transmitted provided always. that the speed oi scanning is such that the reconstituted image appears continuous owing to the retentivity of vision.

Figure 3 shows the phase relationship that has to be maintained between the ,two scanning discs. In this figure the apertures 38, 40 and 42, are on the disc, at (say) the near station and the apertures (shaded in the drawings, in, order to distinguish them) 44, 46 and 48,

" are on the disc at the far station. It will be seen that the corresponding apertures 40 and 46 occupy similar positions in their respective areas 34 and 36. The effect produced by a departure vfrom this phase relationship is il1ustrated in Figure 4; in this diagram the tar disc is shown in advance of the near disc so that when the aperture 40 comes into the area 34 the aperture 46 will already be a certain distance in the area 36 and when the aperture 46 goes out of the area 36, the aperture 40 will still be a distance in'the area 34. The result is that the image of the operator at the far station will be displaced to the left of the area 34 on the near station disc, and a black band will appear at the other edge of the picture, while at the far station the image of the near operator will be displaced to the right of the area 34 and a black band will appear at the left side of the area 34. If the discs get sufllciently out of phase the image of the far operator will pass, towards the left-hand side,,. right out of the area 34 at the near station leaving a blank space while the image of the near operator will pass right out of the area 34 at the far station by way of the right-hand side.

The image signals are applied to the line 50, by means or the transformers 52 and 53, at the near and far stations respectively. I The input terminals .of the valves 54 and 56 situated at,

the near and far stations respectively, are also connected across the line 5. The grid electrodes of these valves being so biased that the valve 54, is working on the bottom bend of its characteristic while the valve 56, is working on the top bend of its characteristic.

The anode circuits of these valves each contain a transformer that feeds the corresponding neon-glow lamp amplifier. Now,.the image; signal impulses are put on to the line in such a sense that those from the near station cause the upper conductor shown in the figure (and consequently the grids of the two valves 54 and 56) to become negatively charged relative to the lower conductor and the valve filaments. The result isthat-the impulses create corresponding impulses in theanode circuit of the distant valve 56, buthave no effect on the anode current of thenearvalve 54, since the grid is already negatively biased so that the valve is at the bottom bend of its characteristic. Alternatively a commutator, arranged upon the shaft of the motor drivingthe scanning device,

may disconnect the local glow lamp amplifier when the local image municating line.

It will be appreciated that. it is desirable,

In the'specificatiqn. of British Letters Patent No. 288,882 isdescribed means whereby a person sitting 'in darkness may be viewed by an in the case of spotlight scanning, to cut out the. visible components of the scanning beam and to signals are on the comapparatus using invisible rays, this apparatus consisting of an exploring and a viewing device mounted upon the same rotating member.

This particular device is eminentlysuitable forf two-way television, one of each'of these devices being placed atthe sending and receiving station and rotated synchronously.

1 In connection with all the arrangements hereinbei'ore described, it will be appreciated by those skilled in the art, that appropriate measures must be taken in order to ensure that with any particular arrangement the reconstituted image of the object at the distant station por-.

trays that object not only the right way up, but

150v also without lateral inversion. This end l usually accomplished in any given case'by -ap-- propriately choosingthe direction 01. throw of corresponding transmitting and receiving spirj' rals, or by inserting an inverting lens or prism or pair of mirrors in some appropriate position in the path of the scanning or image-torming-- rays, or in cases where two geared or otherwise coupled mechanisms are used, one for scanning and the other for reconstituting,-the sense of rotation must be fixed by consideration otthe system as a whole.

It should be noted also that either vertical orhorizontal strip-scanning) may be' employed,

though in a preferred embodiment accordingto the present invention, Ipurpose to employ horizontal scanning in order that when a single disc provided with two concentric spirals isem--' ployed at each station, using the method of'exploration. technically known as spotlight scanning, in which a scanning or projection lens is interposed in the path of the scanning rays, the image .reconstituted at the receiver bymeans of a'flat-plate neon tube behind the discmay be reproduced the right way up by choosing the throw of the receiving spiral in the opposite sense to that of the transmitting spiral, so that lateral inversion is obviated. This is clear on detailed consideration of the system, for suppose the observers see the scanning apertures moving from right to left, then, owing to the scanning or projection lens above mentioned, the scanning light spots move from left to right; thus lateral inversion does not exist, since the neon plates are also exposed from their left to their right sides. It is, of course, necessary to choose 'the throw of the transmitting and receiving spirals in the opposite sense, so that if the receiving apertures move from top to bottom of the image, the transmitting apertures must move up, so that, owing to the projection lens, the scanning light spot moves down, that is from top to bottom of the object.

It will be obvious to any person skilled in the art than any other convenient form of scanning or reconstituting device might be employed in place of discs with spirally arranged -lighttransmitting devices.

For instance, in another embodiment according to the present invention, we may employ two mirror-drums driven by the same motor, one for scanning the object and theother for reconstituting an image, at each station. Obviously, the same method of synchronizingand of'laterally shifting the scanning beam on to the observer's face while he is looking at the reconstituted image is applicable. a

I may also employ apertured or lensed drums or truncated cones for scanning and'for reconstitution.

In alternative modifications, it may be more convenient to employ floodlighting instead of spotlighting for scanning the object at each station.

An additional advantage of the system of twoway television, hereinbefore described, is that when a single disc is employed with two spirals of apertures, by appropriate arrangement of the maskusually employed tolimit or frame the received picture -for instance, by enlarging its dimension in the direction of rotation of the disc to double that value which is required to expose only the picture received from the opposite station a picture of the observer's own person may be presented to him, side by side (in the case of horizontal scanning) with that of the distant observer. i

Having described my invention, what I claim as new and desire to secure by Letters Patent 1. In or for a two-way television system means" for maintaining syntony between the scanning mechanisms at the two stations comprising a scanning disc at eachstation having apertures arranged to give intermittent scanning and a phonic wheel at-each station controlling the rotation of the-said scanning discs and fed with the combined signals generated by the said discs.

2. In or for a two-way television system means for maintaining syntony between the scanning mechanisms at the two stations comprising a scanning disc at each station having apertures arranged to give interrupted scanning and a tation of the said scanning discs and fed with a polarizing direct current upon which is superimposed the combined signals generated by scanning with the said discs.

' 3'. A duplex television system consisting intwo of spirally arranged apertures interdigitated to scan groups of strips of the image. Of the observerand to reconstitute groups of stripsof the image of his co-operator alternately; the said light sensitive cell and light. valve being connected to the channel in such a sense that local signals coming from the light sensitive cell and distant signals going to the light valve are out of phase as regards a principal component of their waveforms.

JOHN LOGIE BAIRD.

100 phonicwheel at each station controlling the ro- 

